Headspace Dimensions & Gauges for .308 ARs

[Benito]

Disregard - I'll wait and see how the thread shakes out.
Benito,
Have you purchased the .308 headspace gauges yet? If you did where did you pick them up?

Not yet.
Was about to from Brownells, but looking into the Hornday tool that Bret posted about.

[Benito]

Thanks for the info. I am looking into this.

My end goal is to check/confirm that my build headspaces properly, is safe and will feed reliably.

I'm still trying to figure out what your end goal is. There doesn't have to be an end goal. I sometimes look at things just to figure out how they work, but buying headspace gauges isn't cheap and won't really do anything for you other than confirm that your rifle is properly headspaced. It almost certainly will be properly headspaced, especially if you're not having any issues.

I think I'll take the liberty of suggesting a tool that I've found to be very useful. When you're firing ammo in a particular rifle, you're not really concerned about the actual headspace of the rifle. It's the difference between the rifle's headspace and the cartridges headspace (measured from the base of the case to the center of the shoulder) that matters. The cartridge's headspace has to be smaller or else you'll have trouble with it chambering. If you reload, you ideally want to set things up so that the cartridge headspace is 0.001" to 0.002" less than the rifles actual headspace. For a semiauto, you want this measurement to be a more generous 0.003" to 0.005". The reason you need a greater difference with semiauto cartridges is that you have to allow for debris and the fact that the bolt usually does not have as much camming action. Now if you don't have headspace gauges, how do you measure a rifles headspace and compare it to a cartridges headspace? You start with a case that's been fired in the rifle in question. Then you use the Hornady Lock-N-Load Headspace tool on a set of calipers to measure the headspace of the fired case. The headspace of this case will be right at 0.001" less than you're rifles actual headspace due to brass contraction. The really cool thing about this tool set is that you can use it to measure headspace of virtually all bottle necked rifle cartridges.
https://www.youtube.com/watch?v=P-UrMTyJ1_E

http://www.hornady.com/store/Lock-N-...h-Body-1-Each/

So if you don't reload, how does this tool benefit you? Well, you can compare the difference in your rifles headspace (measured from a fired case) to that of new cartridges. Ideally, you want that same 0.003" to 0.005" difference for your AR10. All else being equal (key phrase), a rifle & cartridge combination that's within this difference will be more accurate than one where there's a greater difference. One thing to remember is that rifle manufacturers often headspace their rifles on the high end of the range so that they can be confident all commercially available ammo will chamber. Ammo manufacturers often headspace their cartridges on the low end of the range so that they can be confident their ammo will chamber in rifles that have headspace of the low end of the range. Combine both of these together and there can be way more of a difference than you'd want. Keep in mind that there's more to accuracy than headspace, bullets and powder come to mind first, so measuring headspace is just one part of the equation.

The bottom line is that I think this tool will be much more useful for you than a set of headspace gauges for probably about the same money.

[RobertTheTexan]

Thanks for the info. I am looking into this.

My end goal is to check/confirm that my build headspaces properly, is safe and will feed reliably.

I haven't checked headspace of any AR's I've built so far. I'm not saying that it shouldn't be checked, just that I haven't been performing that test. In some builds, I've used the same manufacturer for barrel and BCG, Daniel Defense, Ballistic Advantage for example, but by and large my most common config is a BA barrel with an LMT BCG.
I haven't ran into any issues with my AR-15's that I would relate to headspace. The only issue I encountered was resolved by using an A5H4 buffer.
However with my 308 build, I have been thinking more and more about headspace mainly because I don't have that same mil spec warm and fuzzy I have with my AR-15's


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[Bret]

I agree, but I use the RCBS Precision Case Micrometer to do something similar.

I think the primary advantage that the Hornady tool has over the RCBS Precision Case Micrometer is that it works for almost all bottle necked cartridges. I've never used one, but from what I understand you have to buy a separate RCBS tool for each cartridge. If I was going to only use it for one cartridge, I'd buy the RCBS tool, but since I have a good number and want more, I use the Hornady tool.

The only thing I'd take issue with is using a fired case to determine headspace. That works in a bolt rifle, but not necessarily in a semi-auto. I've never tried it in an AR-10, but in a FAL you get wildly different headspace measurements between actual rifle headspace and fired cases (both with the gas on and turned off).

No doubt it's not as exact as I'd hope and more enters in to it than I wrote above, but I was trying to keep it simple. Fired cases from different manufacturers will not always measure the same due to differences in what makes up the brass and the thickness of the brass. You also have to be careful that the fired case heads are not deformed at all but are completely flat.

It's funny that you should mention the FAL. It was the first rifle that I used the Hornady tool with back when I started reloading. Actually, mine was made by Stoney Point before Hornady bought them. My factory Brazilian Springfield Armory SAR4800 was incredibly inaccurate. Not only that, it mangled cases like crazy. When I used the Hornady tool to make measurements of fired cases, I found differences of up to 0.020". Keep in mind that these measurements should pretty much be the same. This was before the internet, so I was really on my own. No matter what I did, the rifle was inaccurate. I even got bright rings around a few fired cases from factory loaded ammunition. It then dawned on me that the headspace might be excessive. I couldn't reuse the FAL fired cases because they were so all over the place, so I used some other fired cases and backed out my sizing die. As a result, the difference between before and after firing reduced significantly. However, accuracy didn't improve much. After some more measuring, I found that the before and after firing diameters of the cases were significantly different too. The bottom line is that that FAL had a very enlarged chamber. A few years later I decided to build an STG58 FAL. I followed the instructions in the video that I got from Gunplumber and used a set of Forester 308Win gauges to headspace the STG58. The STG58 shot much fore accurately than the factory built SAR4800. As a bonus, it doesn't mangle brass and the headspace as measured from fired cases is consistent. What this taught me is that if a chamber has excessive headspace or is significantly large in diameter, the fired cases will not always be able to fully expand in the chamber. In such a case, you can't use fired cases as a tool to estimate the headspace. If a chamber is this excessive, then there's not much that can be done from an accuracy standpoint anyway. I sold the SAR4800 and still have the STG58 that I built.[/QUOTE]

[T2C]

I agree, but I use the RCBS Precision Case Micrometer to do something similar. The only thing I'd take issue with is using a fired case to determine headspace. That works in a bolt rifle, but not necessarily in a semi-auto. I've never tried it in an AR-10, but in a FAL you get wildly different headspace measurements between actual rifle headspace and fired cases (both with the gas on and turned off).

If you reload, you can bump the shoulder on your case back 0.001" at a time until the bolt just barely closes to determine the actual headspace of the rifle. You have to be very gentle when closing the bolt or you'll compress the brass and introduce an error in your measurement.

That has been my experience with brass fired in Garands and M1As. Brass datum line to base measurements were longer than chamber headspace gauges that would not allow the bolt to close.

Interestingly enough, once fired brass from my Rock River Arms LAR-8 dropped right into a Wilson .308 case length headspace gauge. I would not consider that a recommendation to assume .308 AR brass will not stretch during the ejection cycle though.

[Bret]

It could be that some semiauto rifles begin to unlock before the cases have fully expanded.

[Benito]

I haven't checked headspace of any AR's I've built so far. I'm not saying that it shouldn't be checked, just that I haven't been performing that test. In some builds, I've used the same manufacturer for barrel and BCG, Daniel Defense, Ballistic Advantage for example, but by and large my most common config is a BA barrel with an LMT BCG.
I haven't ran into any issues with my AR-15's that I would relate to headspace. The only issue I encountered was resolved by using an A5H4 buffer.
However with my 308 build, I have been thinking more and more about headspace mainly because I don't have that same mil spec warm and fuzzy I have with my AR-15's


Sent from my iPhone using Tapatalk

Agreed.
AR-15 components are pretty certain to be compatible. 308 ARs, not so much, only components made by the same manufacturer

[Matt in TN]

No doubt it's not as exact as I'd hope and more enters in to it than I wrote above, but I was trying to keep it simple. Fired cases from different manufacturers will not always measure the same due to differences in what makes up the brass and the thickness of the brass. You also have to be careful that the fired case heads are not deformed at all but are completely flat.

I figured you knew this, but thought the OP might not understand this detail and need some clarification. I too put way too much time and energy into measuring cases on a FAL and nitpicking headspace - it'll make you crazy!